Generally, heat-recoverable, or heat-shrinkable, articles are composed of cross-linked polymers. When such materials are exposed to heat at or in excess of their crystalline transition temperatures they may be expanded and cooled in that expanded configuration to create a heat recoverable article. In the alternative, if the article was previously stretched and cooled, it is rendered dimensionally heat-unstable and upon exposure to heat at or in excess of the crystalline transition temperature, the material recovers to its dimensionally heat-stable configuration, i.e. the unstretched configuration. As is readily appreciated by those of ordinary skill in the art, there are many such materials, polymeric or not, exhibiting a large range of crystalline transition temperatures which are selected to suit a particular purpose based upon the physical characteristics of both the material and its crystalline temperature.
The prior art relating to the expansion of heat-recoverable articles deals only with the expansion of heat recoverable articles that are not temperature auto-regulated or self-heating. Whether the article is a flat sheet, a tube, or other shape, the article is first heated, then stretched and then cooled while still in tension so that the article retains the stretched shape until it is heated sufficiently to cause it to recover to its relaxed shape.
In the prior art the recoverable material is placed typically in a hot glycerin bath to soften it prior to stretching it. For small pieces or short lengths of tubes the stretching process is generally performed manually with the operator manually placing the article into and removing it from the glycerin bath. Once the recoverable article is up to temperature, the stretching process is performed by placing the article in a stretching jig or by pushing a mandrel into the tube, and then cooling the article below the crystalline temperature while the stretching mechanism is still in place. The stretching operation can either be begun with the article still in the glycerin bath or immediately after it has been removed from the bath. If the stretching is performed after the article is removed from the bath, the stretching may be uneven since the article may cool unevenly due to remaining drops of hot glycerin on or in the article when it is removed from the bath. Additionally, the article may also be stretched in a direction other than the direction or directions opposite to the direction or directions in which the recovery is desired upon reheating, i.e. the length of a tube into which a mandrel is inserted may have its length increased which will result in an unpredictable reduction in its length when recovery is initiated.
Additional problems occur due to non-uniform heating of the mass of the recoverable material in the heat-recoverable article. Thus, in heat-recoverable articles in which the heat-recoverable material is of an even thickness or even mass distribution, the problem has been to obtain uniform or even heating of the heat-recoverable material to assure uniform recovery. In other articles wherein the thickness or mass of the heat-recoverable material differs in various parts of the heat-recoverable article, the problem has been to obtain appropriate heat distribution and sufficient heating in each of the various areas of different thickness or different mass in order to achieve sufficient recovery of the higher mass areas without overheating the areas of lower mass.
U.S. Pat. No. 3,253,618 ("Cook") discloses that a heat-shrinkable tube of elastic memory material reinforced with braided fibers exhibits an undesirable change in length when the diameter of the tube is reduced by shrinkage. Cook points out that an increase in length upon shrinkage should be kept to a minimum in the case of splices for electric cables due to the limited space available for such axial extension. To minimize such axial extension, Cook provides a tubular shape with reinforcement on an inner surface, an outer surface or as an internal reinforcement. The reinforcement comprises two knitted layers, one being a left-hand stitch and the other being a right-hand stitch to offset the twisting tendency of each layer. The reinforced tube is heated, expanded and cooled to provide an elastic memory material. To provide maximum radial and minimal axial dimensional change, the angle between the axis of the knit and axis of the tube should be 15.degree. to 19.degree.. The heating is performed by passing the reinforced tube through a tank containing a heating liquid such as glycerine at 295.degree. F. and the expanding is performed by applying pressure such as a 5 psi pressure differential to the interior of the tube. The knitted reinforcement prevents the tube from shrinking axially more than 10% during the expansion or during heat shrinkage of the tube.
U.S. Pat. No. 3,597,372 ("Cook") discloses several methods of manufacturing heat-recoverable thermoplastic materials. According to one method, a considerable amount of built-in stress is imparted during fabrication and the material is quenched to hold the molecules in the stressed condition. In the case of cross-linked polyethylene, the material is heated above the crystalline melting temperature so the material behaves as an elastomer, the material is deformed while in the elastomeric state and the temperature of the material is reduced to cause crystallization which maintains the cross-linked polymer in its deformed condition. Another method involves heating and applying an external force to deform a heat recoverable elastomeric article into a desired heat-recoverable configuration, quenching the deformed article and then releasing the external force. The elastomeric article can be an uncured elastomer in which a normally solid, heat-flowable material such as a thermoplastic or non-elastic resinous material is incorporated. Alternatively, the elastomeric article can be a plasticized thermoplastic material such as a plasticized polyvinyl chloride. After the elastomeric article is formed, it is vulcanized or cross-linked. The heating is at a temperature at which the thermoplastic or resinous material loses the major portion of its strength. The elastomeric article can be a heat-shrinkable tubing or a heat-expandable tubing. To produce the heat shrinkable tubing, tubing is heated in a hot glycerine bath and a heated mandrel is used to expand the tubing after which the tubing is cooled in water while over the mandrel. To produce a T-shaped 3-finger splice for use in a cable harness, a hollow T-finger shaped article is molded, the article is immersed in glycerine at 150.degree. C., the finger orifice is mechanically expanded to a desired dimension and the article is cooled in water.
U.S. Pat. No. 4,035,534 ("Nyberg") discloses a process of preparing a composite tube wherein a heat-recoverable tube adhesively bonded to an inner or outer elastomeric tube is heated and expanded on a mandrel, cooled and the mandrel is withdrawn. Alternatively, the heat-shrinkable tube is heated and expanded on a heated mandrel and the inner surface of an elastomeric tube is bonded to the heat-shrinkable tube either before or after cooling the heat-shrinkable tube. The elastomeric tube can be an inner layer which includes conductive particles such as carbon black or metal particles.